1. Technical Field
This invention relates to cordless telephones and more particularly to frequency selecting arrangements for selecting an available channel for communications between a handset unit and a base unit of a cordless telephone.
2. Description of the Prior Art
Cordless telephones have been allocated a limited number of communications channels over which they may operate. In the United States, for example, there are only ten allocated channels. And as cordless telephones increase in popularity so does the likelihood of interference between these telephones which must all communicate on these few channels. Although the ten cordless telephone communication channels are available, most cordless telephones in use today operate over only one, two or three of these channels. These telephones operate over a selected channel until the user takes some action to change this channel.
The user achieves a channel change for his or her cordless telephone either by selecting, at the handset unit or base unit as appropriate, a different one from this limited set of channels for operation or, in the case of a one-channel cordless telephone, by returning the telephone to the supplier and obtaining a cordless telephone that operates on a different channel. When the user of such a cordless telephone begins the process of placing a telephone call, there is no way for the user to determine beforehand if the channel on which the cordless telephone will operate is then in use by, for example, another cordless telephone in the vicinity. Thus when the user obtains dial tone from the base unit it may or may not be subjected to interference from another user of a cordless telephone or even some other form of radiation on this channel. And the probability of interference is proportional to the number of other cordless telephones within the reception range of the user's cordless telephone and the length of time each of these telephones is in use over a given period of time.
Recently, some cordless telephones are now being built that are capable of selecting a channel on which to operate from any of the allocated channels. Although this increases the likelihood of the user finding an interference-free channel, he or she may still have to change channels a number of times before finding this channel, all the while causing interference to other users with his or her cordless telephone while searching for an available channel.
In an effort to minimize interference with other cordless telephones, one arrangement described in U.S. Pat. No. 4,768,219 provides a channel scanning process for selecting an unoccupied channel. In this arrangement, the handset and the base unit while in a standby mode scan the allocated channels for detecting the ones with interference. When communications is desired by a user, the telephone is placed in an active mode where the base and handset both scan the channels that were not found to have interference. Although this arrangement eventually selects an available channel, it has the disadvantage of being slow to respond to a user attempting to use the telephone as the base and handset attempt to locate each other on a mutually acceptable channel. This arrangement may also cause interference to other users as the base or handset transmits on what are believed to be free channels while trying to locate each other on the mutually acceptable channel.